The Kuo Toans of Blibdoolploop hate other humanoid races, using them as slaves and food.
The Kuo Toans of the Shrine worship Blibdoolpoolp who resides in the Elemental Plane of Water.
The Kuo Toa captured a drow scouting party led by Derinnil d’Noquar. She is the only one remaining and is being prepared for ritual disembowelment.
The ziggurat of the kuo toa hides a monolith beneath tied to the Elemental Plane of Water and one of the ancient monoliths of the Black Cathedral.
Priest-Prince Va-Guulgh leads the kuo toa of their ziggurat. He is vain and bloodthirsty. He believes he is a chosen child of Blibdoolploop.
Va-Gullgh is served by his Master of Assassins – Quolp-Ool.
A continual cold war exists in Erelhei-Cinlu. House Eilservs rules the other houses of the city, but is constantly threatened by the other houses. Matron mother Eclavdra dEilservs is the matron mother of Erelhei-Cinlu.
Several gods and demon princes seek to use the Black Cathedral as their conduit to Oerth – Orcus, Demogorgon, Lolth, Blibdoolpoolp the Sea Mother. The Black Carhedral is the battleground of their followers.
Dearest sisters, your father, King Thrael, is dead, as is your elder brother. Baron Elinsar claims the throne, having used Hawkwind's vile blade to his ends, and civil war rages in Celine. Do not come home. Even in most ancient texts only touch upon this Black Cathedral and do not speak of it well. They speak of it as a conduit of fell gods seeking to be closer to the mortals who fuel them. If you can close those unfollowed doors, all of Oerth may be saved. But be careful or you may lose yourself. I shall always be loyal to you, my Queens. – Siannodel, Master of Lore
Each destroyed obelisk tips the balance of chaos surrounding the Black Cathedral. Only the souls of the dead can solidify it and crack open its massive doors.

Notable NPCs

Thoopshib the Kuo Toa Bargemaster – Likes gemstones
Quolp-Ool – Master of Assassins
Derinnil d’Noquar — captured drow scout
Priest-Prince Va-Guulgh — Kuo Toa leader

Monsters

Kuo Toans CR 1/2
Kuo Toa Monitors CR3
Kuo Toa Assassin Priests CR6
Giant Lizards CR3
Avatar of Blibdoolpoolp CR 12

Treasure

114 SP, 2346 GP, 119 PP
3 × 100 GP garnet gemstones
200 GP shadowy gemmed gold crown
Nightmarish spiked lamp that casts Magic Jar once
- Chime of Opening
- +2 ethereal mace

Session Notes

Seeking the stone of the frozen sea
Killed a whole bunch of Kuo Toa
Heard the voice of the drow prisoner to the west

Two two-headed crocodile approach our boat as it crosses the river. We defeat them and make it across the river. As she is cleaning one of crocs, Chartreuse sees a sigil of the black harbinger, and the kou toa makes it clear that crocs like this are not a common occurance, and we realize that we were targeted. As we are enjoying a long rest, a silver raven lands on Zaffre and delivers her a note that tells her that the assassin that was after them has killed Zaffre and Chartreuse's father and brother at the order of the baron who was Chartreuse's fiance. The baron has claimed the throne. The note tells them that they should continue and close the door to the black cathedral.

We continue toward the kou toa and enter a chamber with a statue/effigy of a crayfish with a bunch of skulls at the base. Three kou toa approach as if to attack, and we kill them. We decide to follow a small river toward our destination. We come across a bunch of stupid poison spears that we manage to avoid. We continue along and approach a large chamber with kou toa, an there are hundreds in the area. Chartreuse pierces the mind of some guards and knows who they fear, and where the central ziggarat/shrine is (Northeast) and that they all fear something down below. Zaffre, still affected by the turmoil in her life (or the bad drawing of crowns and lines) kills them all with a death sphere -- seems like she's working through some things. We continue onward, killing a pilgrim, until we come across 18 guards and 2 monitors. We kill them. Chartreuse hears a drow voice in her head saying, "I guess it couldn't get any worse for me today" and a voice out loud saying "help!" in drow.